Mycorrhizae enhance reactive minerals but reduce mineral-associated carbon

Soil organic carbon (C) is the largest active C pool of Earth's surface and is thus vital in sustaining terrestrial productivity and climate stability. Arbuscular mycorrhizal fungi (AMF) form symbioses with most terrestrial plants and critically modulate soil C dynamics. Yet, it remains unclear whether and how AMF–root associations (i.e., mycorrhizae) interact with soil minerals to affect soil C cycling. Here we showed that the presence of both roots and AMF increased soil dissolved organic C and reactive Fe minerals, as well as litter decomposition and soil CO₂ emissions. However, it reduced mineral-associated C. Also, high-resolution nanoscale secondary ion mass spectrometry images showed the existence of a thin coating (0.5–1.0 μm thick) of ⁵⁶Fe¹⁶O⁻ (Fe minerals) on the surface of ¹²C¹⁴N⁻ (fungal biomass), illustrating the close physical association between fungal hyphae and soil Fe minerals. In addition, AMF genera were divergently related to reactive Fe minerals, with Glomus being positively but Paraglomus and Acaulospora negatively correlated with reactive Fe minerals. Moreover, the presence of roots and AMF, particularly when combined with litter addition, enhanced the abundances of several critical soil bacterial genera that are associated with the formation of reactive minerals in soils. A conceptual framework was further proposed to illustrate how AMF–root associations impact soil C cycling in the rhizosphere. Briefly, root exudates and the inoculated AMF not only stimulated the decomposition of litter and SOC and promoted the production of CO₂ emission, but also drove soil C persistence by unlocking mineral elements and promoting the formation of reactive minerals. Together, these findings provide new insights into the mechanisms that underlie the formation of reactive minerals and have significant implications for understanding and managing soil C persistence.     

Long-Term Fertilization Modifies the Structures of Soil Fulvic Acids and Their Binding Capability with Al

The binding characteristics of organic ligands and minerals in fulvic acids (FAs) with Al are essential for understanding soil C sequestration, remain poorly understood. In this study, Fourier transform infrared (FTIR) spectroscopy combined with two-dimensional correlation spectroscopy (2DCOS) analysis was applied for the first time to explore the binding of Al with organic ligands and minerals in soil FAs. For these analyses, two contrasting treatments were selected from a long-term (i.e., 22-year) fertilization experiment: chemical (NPK) fertilization and swine manure (SM) fertilization. The results showed that the long-term application of organic and inorganic fertilizers to soils had little effect on the compositions of the fluorescent substances and organic ligands in the soil FAs. However, long-term SM fertilization increased the weathered Al and Si concentrations in the soil FAs compared with long-term chemical fertilization. Furthermore, organic ligands in the soil FAs were mainly bound with Al in the NPK treatment, whereas both organic ligands and minerals (Al-O-Si, Si-O) were bound with Al under the M fertilization conditions. Both transmission electron microscopy (TEM) images and X-ray diffraction spectra demonstrated that amorphous and short-range-ordered nanominerals were abundant in the soil FAs from the SM plot in contrast to the soil FAs from the NPK plot. This result illustrates the role nanominerals play in the preservation of soil FAs by during long-term organic fertilization. In summary, the combination of FTIR and 2D correlation spectroscopy is a promising approach for the characterization of the binding capability between soil FAs and Al, and a better understanding FA-Al binding capability will greatly contribute to global C cycling.     

Effects of hyper-enriched reactive Fe on sulfidisation in a tidally inundated acid sulfate soil wetland

Solid phase Fe and S fractions were examined in an acid sulfate soil (ASS) wetland undergoing remediation via tidal inundation. Considerable diagenetic enrichment of reactive Fe(III) oxides (HCl- and dithionite-extractable) occurred near the soil surface (0?C0.05 m depth), where extremely large concentrations up to 3534 ??mol/g accounted for ~90% of the total Fe pool. This major source of reactive Fe exerts a substantial influence on S cycling and the formation, speciation and transformation of reduced inorganic S (RIS) in tidally inundated ASS. Under these geochemical conditions, acid volatile sulfide (AVS; up to 57 ??mol/g) and elemental sulfur (S⁰; up to 41 ??mol/g) were the dominant fractions of RIS in near surface soils. AVS?CS to pyrite?CS ratios exceeded 2.9 near the surface, indicating that abundant reactive Fe favoured the accumulation of AVS minerals and S⁰ over pyrite. This is supported by the significant correlation of poorly crystalline Fe with AVS?CS and S⁰?CS contents (r = 0.83 and r = 0.85, respectively, P < 0.01). XANES spectroscopy provided direct evidence for the presence of a greigite-like phase in AVS?CS measured by chemical extraction. While the abundant reactive Fe may limit the transformation of AVS minerals and S⁰ to pyrite during early diagenesis (~5 years), continued sulfidisation over longer time scales is likely to eventually lead to enhanced sequestration of S within pyrite (with a predicted 8% pyrite by mass). These findings provide an important understanding of sulfidisation processes occurring in reactive Fe-enriched, tidally inundated ASS landscapes.     

Investigating the effect of ascorbate on the Fe(II)-catalyzed transformation of the poorly crystalline iron mineral ferrihydrite

The inorganic core of the iron storage protein, ferritin, is recognized as being analogous to the poorly crystalline iron mineral, ferrihydrite (Fh). Fh is also abundant in soils where it is central to the redox cycling of particular soil contaminants and trace elements. In geochemical circles, it is recognized that Fh can undergo Fe(II)-catalyzed transformation to form more crystalline iron minerals, vastly altering the reactivity of the iron oxide and, in some cases, the redox poise of the system. Of relevance to both geochemical and biological systems, we investigate here if the naturally occurring reducing agent, ascorbate, can effect such an Fe(II)-catalyzed transformation of Fh at 25?°C and circumneutral pH. The transformation of ferrihydrite to possible secondary Fe(III) mineralization products was quantified using Fourier transform infrared (FTIR) spectroscopy, with supporting data obtained using X-ray absorbance spectroscopy (XAS) and X-ray diffraction (XRD). Whilst the amount of Fe(II) formed in the presence of ascorbate has resulted in Fh transformation in previous studies, no transformation of Fh to more crystalline Fe(III) (oxyhydr)oxides was observed in this study. Further experiments indicated this was due to the ability of ascorbate to inhibit the formation of goethite, lepidocrocite and magnetite. The manner in which ascorbate associated with Fh was investigated using FTIR and total organic carbon (TOC) analysis. The majority of ascorbate was found to adsorb to the Fh surface under anoxic conditions but, under oxic conditions, ascorbate was initially adsorbed then became incorporated within the Fe(III) (oxyhydr)oxide structure (i.e., co-precipitated) over time.     

Seasonal cycling of Fe in saltmarsh sediments

This study combines an analysis of porewater chemistry with new, solid phase wet chemical extractions to examine the seasonal cycling of Fe in vegetated and unvegetated (cyanobacterial mat) saltmarsh sediments. Saltmarsh sediments are shown to contain more solid phase reactive Fe than other marine sediments studied so far. From the partitioning and speciation of solid Fe, and solid/soluble reduced S analysis in 10 sediment cores, we have observed that a majority of solid Fe in these sediments is cycled rapidly and completely between oxidized reactive Fe and reduced Fe as pyrite. Vegetated porewaters showed a lower pH and much higher Fe(II) concentrations on average than unvegetated porewaters in the top 10 cm, whereas sulfate, alkalinity, and sulfide concentrations were similar in the two environments. The amorphous Fe(III) oxide fraction showed a high negative correlation to solid and soluble reduced S (r ² = –0.86 and –0.71, respectively) in surface vegetated sediments whereas the crystalline Fe(III) oxide fraction showed a high negative correlation (r ² = –0.96) to sulfide only at depth. Though reactive Fe was observed in unvegetated sediments, no seasonal trend was apparent and the speciation of solid Fe revealed that most of it was reduced. Solid phase and porewater chemistry support the dominant role of the biota (Spartina alterniflora and bacteria) in controlling the reactivity of Fe and suggest that the current definition of solid phase, reactive Fe should be expanded to include crystalline Fe(III) minerals which are available for pyrite formation in saltmarsh sediments. In support of previous saltmarsh studies, we present evidence that the redox cycle of solid Fe is controlled by sulfate reduction and sediment oxidation which respond to both annual cycles (light, temperature) and to short-term, episodic effects such as weather and tides.     

Association with pedogenic iron and aluminum: effects on soil organic carbon storage and stability in four temperate forest soils

Soil organic carbon (SOC) can be stabilized via association with iron (Fe) and aluminum (Al) minerals. Fe and Al can be strong predictors of SOC storage and turnover in soils with relatively high extractable metals content and moderately acidic to circumneutral pH. Here we test whether pedogenic Fe and Al influence SOC content and turnover in soils with low Fe and Al content and acidic pH. In soils from four sites spanning three soil orders, we quantified the amount of Fe and Al in operationally-defined poorly crystalline and organically-complexed phases using selective chemical dissolution applied to the soil fraction containing mineral-associated carbon. We evaluated the correlations of Fe and Al concentrations, mean annual precipitation (MAP), mean annual temperature (MAT), and pH with SOC content and ¹⁴C-based turnover times. We found that poorly crystalline Fe and Al content predicted SOC turnover times (p < 0.0001) consistent with findings of previous studies, while organically-complexed Fe and Al content was a better predictor of SOC concentration (p < 0.0001). Greater site-level MAP (p < 0.0001) and colder site-level MAT (p < 0.0001) were correlated with longer SOC turnover times but were not correlated with SOC content. Our results suggest that poorly crystalline Fe and Al effectively slow the turnover of SOC in these acidic soils, even when their combined content in the soil is less than 2% by mass. However, in the strongly acidic Spodosol, organo-metal complexes tended to be less stable resulting in a more actively cycling mineral-associated SOC pool.     

Spectral Exploration of Calcium Accumulation in Organic Matter in Gray Desert Soil from Northwest China

Little attention has been paid to the accumulation of soil organic matter (SOM) in the fringes of the mid-latitude desert. In this paper, soil samples from a long-term field experiment conducted from 1990 to 2013 at a research station in Urumqi, China by different fertilizer treatments, were used to determine soil properties and soil dissolved organic matter (DOM) by chemical analysis, fluorescence excitation emission matrix (EEM) spectroscopy, and high resolution-transmission electron microscopy (HR-TEM). The binding features of DOM under the addition of Ca²⁺ were analyzed using a two-dimensional (2D) Fourier transform infrared (FTIR) spectrometer further to explore the response of the DOM to increasing concentrations of Ca²⁺. Long-term application of chemical fertilizers and goat manure increased soil organic carbon (SOC) by 1.34- and 1.86-fold, respectively, relative to the non-fertilized control (8.95g.kg^-1). Compared with the control, application of chemical fertilizers and manure significantly increased the concentrations of Ca, Mg, Si, humic and fulvic acid-like substances in DOM but decreased the amounts of trivalent metals (Al and Fe) and protein-like substances. Although crystalline Al/Fe nanoparticles and amorphous or short-range-order Si/Al nanoparticles existed in all DOM samples, crystalline Ca/Si nanoparticles were predominant in the samples treated with goat manure. Although organic matter and Si-O-containing nanoparticles were involved in the binding of Ca²⁺ to DOM, application of chemical fertilizers weakened Ca²⁺ association with components of the amide II group (1510 cm^-1) and Si-O linkage (1080 cm^-1), whereas application of goat manure enhanced the affinity of Ca²⁺ for Si-O linkage. Our results suggested that the enrichment of Ca in gray desert soil possibly helps accumulate SOM by forming crystalline Ca/Si nanoparticles in addition to Ca²⁺ and organic matter complexes.     

草酸对土壤胶体与矿物表面酶的吸附及活性影响

采用平衡批处理法,研究了模拟根系分泌物--草酸溶液的浓度、pH对酸性磷酸酶在针铁矿、高岭石及黄棕壤和砖红壤胶体(＜2μm)上的吸附及比活的影响.结果表明,针铁矿对磷酸酶的吸附量受草酸浓度的影响较小,其它供试胶体对蛋白的吸附量随草酸浓度的升高,一般表现为先急剧降低(0～5mmol·L^-1),之后逐渐升高到与对照相当或略低.这与草酸在土壤胶体和矿物表面的配位形态及其对载体表面的电荷改变、溶解有关.草酸体系中,供试胶体对磷酸酶的吸附顺序为针铁矿>黄棕壤＞高岭石＞砖红壤.酶在草酸体系中的最大吸附点位一般出现在蛋白的等电点(IEP)和供试胶体的PZC之间,而酶在草酸体系中被固定到供试胶体上之后,其最适比活点随胶体类型的不同而没有变化或有所高移.     

Application of LIBS in Detection of Antihyperglycemic Trace Elements in Momordica charantia

The present study exploits the information based on concentration of trace elements and minerals in understanding the role/mechanism of action of freeze-dried fruit powder suspended in distilled water of Momordica charantia (family: Cucurbitaceae) in diabetes treatment. Laser-induced break down spectroscopy (LIBS) spectra of plant product was recorded under optimized experimental conditions and analyzed. Several atomic lines such as Na, K, Mg, Ca, Fe, Al, etc. have been observed in the LIBS spectra of the above plant product. The concentrations of these minerals are determined by using calibration-free LIBS method. Correlation between the concentration of these elements/minerals and their defined role in diabetes management was studied in normal as well as diabetic animal models.     

Mineral Elements in Root of Wild Saposhnikovia divaricata and Its Rhizosphere Soil

Mineral elements are important components of medicinal herbs, and their concentrations are affected by many factors. In this study, Ca, Mg, Na, K, Fe, Mn, Cu, and Zn concentrations in wild Saposhnikovia divaricata and its rhizosphere soil collected from seven locations at two different times in China were measured, and influences of rhizosphere soil on those minerals in plant were evaluated. The results showed that mean concentrations of eight minerals in plant samples decreased in the order: Ca > Mg > Na > K > Fe > Zn > Mn > Cu, and those in the soil samples followed the following order: Na > Fe > Ca > K > Mg > Mn > Zn > Cu. Mean concentrations of Ca, Na, Mg, and K in plants were higher than those in soils, while higher mean concentrations of the other four minerals were found in soils. It was found that there was a positive correlation of Mg, Na, and Cu concentrations in the plant with those in the soil respectively, but a negative correlation of Mn concentration in plant with that in the soil. Except Ca, K, and Mn, the other five minerals in plant were all directly affected by one or more chemical compositions of soil. The results also indicate that pH value and concentrations of total nitrogen, Mg, Mn, and Cu in soil had significant correlations with multimineral elements in plant. In a word, mineral elements uptake of S. divaricata can be changed by adjusting the soil fertility levels to meet the need of appropriate quality control of S. divaricata.     

High initial soil organic matter level combined with aboveground plant residues increased microbial carbon use efficiency but accelerated soil priming effect

Biogeochemistry - As rock-derived primary minerals weather to form soil, they create reactive, poorly crystalline minerals that bind and store organic carbon. By implication, the abundance of...     

Use of Ferric and Ferrous Iron Containing Minerals for Respiration by Desulfitobacterium frappieri

The potential of Desulfitobacterium frappieri strain G2, which was isolated from subsurface smectite bedding, to participate in iron redox reactions was investigated. Strain G2 can use poorly crystalline Fe(III) oxide, soluble forms of Fe(III) and Fe(III) in the structure of phyllosilicate minerals as electron acceptors. It can also oxidize Fe(II)-NTA or Fe(II) in the structure of phyllosilicate minerals with nitrate as the electron acceptor. These results suggest for the first time that strains of Desulfitobacterium frappieri may play an important role in iron cycling in sedimentary environments.     

Effect of land use change on soil carbon in Hawaii

Organic carbon storage and turnover were altered in soils formed from volcanic ash (Andisols) as a result of conversion of tropical forest to pasture and sugarcane cropland. Changes in soil carbon storage after approximately a century of each land use were estimated using stable carbon isotope values and carbon contents. Total organic carbon stored in soils varied as a result of management, with pasture soils showing net carbon gain and sugarcane soils showing net carbon loss. In pasture soils, increases in carbon at depth (40 to 80 cm) are below the rooting zone of the introduced (C4) vegetation, and have stable carbon isotopic values indicative of forest (C3) plants. Within the pasture rooting zone (0–40 cm) the isotopic data reveals that additions of pasture (C4) organic matter have been offset by losses of C3 carbon. The concentration of Fe/Al oxides (soil minerals that bind with organic matter to form oxide-humus complexes) appear to control the quantity of carbon stored in soils, as well as the difference in the depth and magnitude of carbon storage changes that occur with each type of land use change. Sugarcane land use appears to induce dissociation of Fe/Al oxide-humus complexes and loss of oxide-associated organic matter from the profile. In pastures, Fe/Al oxide-humus complexes are translocated to deeper horizons in the soils, resulting in greater profile carbon storage and longer apparent turnover time of carbon stored below 50 cm depth. In this high precipitation region, carbon losses from the soil appear to occur via downward transport, either as colloids or in solution, in addition to the generally assumed pathway of flux to the atmosphere as CO₂.     

Surface alteration of realgar (As4S4) by Acidithiobacillus ferrooxidans

The chemical and physical characteristics of realgar (an arsenic sulfide mineral that occurs in several crystalline forms) in the presence of Acidithiobacillus ferrooxidans BY-3 were investigated in this work. Grains of the mineral were incubated for 10, 20, and 30 days with A. ferrooxidans cultured in 9K medium at 30 °C and at 150 rpm agitation. Abiotic control experiments were conducted in identical solutions. The effect of bioleaching on the surface properties of realgar was characterized by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), inductively coupled plasma atomic emission spectroscope (ICP-AES), X-ray diffraction (XRD), and Raman spectroscopy. SEM and EDS analyses confirmed the ability of A. ferrooxidans to modify surfaces of realgar and to efficiently enhance its dissolution. ICP-AES showed the dissolution and precipitation of realgar during bioleaching. Based on the XRD pattern and the Raman spectra, the decrease in arsenic in the liquid phase was due to co-precipitation of the mineral with Fe(III) or Fe(III) compounds (e.g., jarosite or goethite). Thus, not only did Fe(III) alter the surface of realgar, but it also promoted its dissolution during bioleaching.     

经营措施对毛竹林土壤不同组分有机碳、氮及化学结构的影响

以次生常绿阔叶林为对照,选择立地条件相近的无经营、粗放经营和集约经营3种类型毛竹林为研究对象,应用密度-粒径联合分组以及化学、生物分析和傅里叶变换红外光谱(FTIR)方法,探讨经营措施对毛竹林土壤不同组分有机碳、氮含量,分配比例及结构特征的影响。结果表明: 与对照相比,无经营和粗放经营毛竹林显著提高了土壤总有机碳(TOC)、全氮(TN)、游离态颗粒有机碳、氮、可溶性有机碳(DOC)、氮(DON)和矿物结合态有机碳、氮的含量。无经营毛竹林虽然显著增大了游离态颗粒有机碳、氮的分配比例,但其与黏土矿物结合的有机碳依然是土壤有机碳的最大贮存库(67.6%)。集约经营导致竹林土壤有机碳、全氮的贮量及各组分有机碳、氮含量下降,但明显增大了DOC/TOC、微生物生物量氮与全氮比值以及微生物生物量碳和土壤有机碳的比值(微生物商)。经营措施对土壤有机碳的化学结构也具有显著影响。与对照相比,无经营和粗放经营毛竹林土壤有机碳中酚醇-OH、脂肪族-CH、芳香族C=C和羰基C=O吸收峰相对强度增强,土壤疏水性显著增加。土壤有机碳的脂肪碳、芳香碳和疏水性与土壤碳氮总量呈显著正相关,与微生物商呈显著负相关。在人为干扰减少的情况下,毛竹林凋落物、根系等有机质残体输入量的增多引起土壤难分解化合物的相对积累,使有机碳化学稳定性明显增强。同时,土壤黏土矿物质对土壤有机碳起到了很好的保护作用,通过矿物-有机碳复合使土壤碳储存稳定性更高,进而有利于土壤碳的长期保存。     

Phosphate minerals in a lateritic crust from Venezuela

Ferruginous crusts and pisolites have chemical and mineralogical properties that differ from the surrounding soil due to Fe and Al enrichments which cause cementations that can harden irreversibly. In addition to, and possibly as a result of the Fe and Al accumulation, other ions, particularly phosphate are often also enriched by a factor of 2–20 relative to the surrounding soil. The P accumulated in ferruginous materials is normally bound to the Fe or Al in amorphous forms of low solubility. Distinct minerals have rarely been identified.We examined a section through a Venezuelan ferruginous crust, which contained portions with P contents>100 g kg^–1, chemically, mineralogically and micromorphologically with the aim to show some of the mechanisms that cause such extreme P accumulation and segregation in a landscape that is otherwise very nutrient poor.Except for the cementation, manifested as an in-filling of pores by Fe, the morphology of the ferruginous crusts reflected that of the original soil. At approx. 30 cm below the crust's surface, goethite, strengite and leucophosphite (KFe₂(PO₄)₂OH·2H₂O) were identified along a downward sequence of pores nearer the surface to pores at greater depth to the matrix of the lower crust. While the lower crust contained highly soluble P, Fe oxides from outer pore spaces showed high P sorption. The element and mineral distribution across thin sections suggested that incoming Fe had interacted with a soil matrix that was exceptionally rich in K and P to form highly soluble leucophosphite, followed by less soluble strengite and finally Fe oxides that essentially occluded the more soluble minerals found in the lower crust. Associated organic C dated at 18,700 y b.p., suggesting that the occlusion process occurred around the last glacial maximum, when the region became more arid. Although extreme in its extent, the process of separation and occlusion of minerals demonstrated here, may be useful for interpreting similar processes in soils and soil cementations that affect the biogeochemical turnover of elements.     

Effects of nitrogen and water addition on trace element stoichiometry in five grassland species

A 9-year manipulative experiment with nitrogen (N) and water addition, simulating increasing N deposition and changing precipitation regime, was conducted to investigate the bioavailability of trace elements, iron (Fe), manganese (Mn), copper (Cu), and zinc (Zn) in soil, and their uptake by plants under the two environmental change factors in a semi-arid grassland of Inner Mongolia. We measured concentrations of trace elements in soil and in foliage of five common herbaceous species including 3 forbs and 2 grasses. In addition, bioaccumulation factors (BAF, the ratio of the chemical concentration in the organism and the chemical concentration in the growth substrate) and foliar Fe:Mn ratio in each plant was calculated. Our results showed that soil available Fe, Mn and Cu concentrations increased under N addition and were negatively correlated with both soil pH and cation exchange capacity. Water addition partly counteracted the positive effects of N addition on available trace element concentrations in the soil. Foliar Mn, Cu and Zn concentrations increased but Fe concentration decreased with N addition, resulting in foliar elemental imbalances among Fe and other selected trace elements. Water addition alleviated the effect of N addition. Forbs are more likely to suffer from Mn toxicity and Fe deficiency than grass species, indicating more sensitivity to changing elemental bioavailability in soil. Our results suggested that soil acidification due to N deposition may accelerate trace element cycling and lead to elemental imbalance in soil–plant systems of semi-arid grasslands and these impacts of N deposition on semi-arid grasslands were affected by water addition. These findings indicate an important role for soil trace elements in maintaining ecosystem functions associated with atmospheric N deposition and changing precipitation regimes in the future.     

Reductive Immobilization of Chromium in Soils Containing Heterogeneous Fe-Bearing Minerals

Cr(VI) immobilization in systems containing Fe-bearing soil minerals was studied in batch and column systems. Batch experiments showed that water chemistry such as solution pH and Cr(VI) concentration had a pronounced impact on Cr(VI) removal by Fe-bearing soil minerals. Acidic conditions were observed to be more favorable for enhanced Cr(VI) removal. The dependence of Cr(VI) removal on Cr(VI) concentration indicated that there were limited numbers of surface sites on Fe-bearing minerals responsible for Cr(VI) removal. A complexing agent, citrate, significantly enhanced both Cr(VI) removal and total Fe-dissolution from the mineral surfaces relative to non-citrate containing systems, and the iron dissolved from the mineral surfaces was in Fe(III) oxidation form, implying that Cr(VI) removal occurred mainly on mineral surfaces, and the surface Fe(II) sites played an active role in Cr(VI) reduction. The results from column experiments showed that the accumulation of surface precipitates resulted in clogging of pore spaces, thereby creating preferential flow paths within the column. However, the addition of citrate significantly prevented the accumulation of surface precipitates due to the formation of highly soluble Fe–citrate complexes. SEM images revealed that the precipitates accumulated in the column had sponge-like shapes. The energy-dispersive spectroscopy analysis provided further evidence that the surface precipitates formed also contained Cr species as well as Fe. Overall it is clear that Fe-bearing minerals may serve as an effective reducing agent for in-situ reductive immobilization of hexavalent chromium in subsurface systems.     

Soil carbon stocks in stable tropical landforms are dominated by geochemical controls and not by land use

Soil organic carbon (SOC) dynamics depend on soil properties derived from the geoclimatic conditions under which soils develop and are in many cases modified by land conversion. However, SOC stabilization and the responses of SOC to land use change are not well constrained in deeply weathered tropical soils, which are dominated by less reactive minerals than those in temperate regions. Along a gradient of geochemically distinct soil parent materials, we investigated differences in SOC stocks and SOC (Δ¹⁴C) turnover time across soil profile depth between montane tropical forest and cropland situated on flat, non-erosive plateau landforms. We show that SOC stocks and soil Δ¹⁴C patterns do not differ significantly with land use, but that differences in SOC can be explained by the physicochemical properties of soils. More specifically, labile organo-mineral associations in combination with exchangeable base cations were identified as the dominating controls over soil C stocks and turnover. We argue that due to their long weathering history, the investigated tropical soils do not provide enough reactive minerals for the stabilization of C input in either high input (tropical forest) or low-input (cropland) systems. Since these soils exceeded their maximum potential for the mineral related stabilization of SOC, potential positive effects of reforestation on tropical SOC storage are most likely limited to minor differences in topsoil without major impacts on subsoil C stocks. Hence, in deeply weathered soils, increasing C inputs may lead to the accumulation of a larger readily available SOC pool, but does not contribute to long-term SOC stabilization.     

Contingency in the direction and mechanics of soil organic matter responses to increased rainfall

Background and Aims

Rainfall is expected to show greater and more variable changes in response to anticipated rising of earth surface temperatures than most other climatic variables, and will be a major driver of ecosystem change.

Methods

We studied the effects of predicted changes in California’s rainy season for storage and stabilization mechanisms of soil organic matter (SOM). In a controlled and replicated experiment, we amended rainfall over large plots of natural grassland in accordance with alternative scenarios of future climate change.

Results

We found that increases in annual rainfall have important consequences for soil carbon (C) storage, but that the strength and even direction of these effects depend critically on seasonal timing. Additional rainfall during the winter rainy season led to C loss from soil while additions after the typical rainy season increased soil C content. Analysis of MIneral-Organic Matter (OM) associations reveals a potentially powerful mechanism underlying this difference: increased winter rainfall greatly diminished the role of Fe and Al oxides in SOM stabilization. Dithionite extractable crystalline Fe oxides explained more than 35% of the variability in C storage under ambient control and extended spring rainfall conditions, compared to less than 0.01% under increased winter rainfall. Likewise, poorly crystalline Fe and Al oxides explained more than 25 and 40% of the variability in C storage in the control and extended spring rainfall treatments, respectively, but less than 5% in the increased winter rainfall treatment.

Conclusions

Increases in annual precipitation identical in amount but at three-month offsets produced opposite effects on soil C storage. Such clear differences in the amount and chemical composition of SOM, and in the vertical distribution of oxides in the soil profile in response to treatment timing carry important implications for the C sequestration trajectory of this ecosystem.